Publish/Subscribe

在RabbitMQ中，producer只能将消息发送到一个exchange中。要理解exchange也非常简单，它一边负责接收producer发送的消息， 另一边将消息推送到queue中。exchange必须清楚的知道在收到消息之后该如何进行下一步的处理，比如是否应该将这条消息发送到某个queue中？ 还是应该发送到多个queue中？还是应该直接丢弃这条消息等等。

RabbitMQ中的exchange类型有这么几种：direct，topic，headers以及fanout。这一小节将会主要介绍最后一种类型——fanout。 使用RabbitMQ的client来创建一个fanout类型的exchange。fanout类型的exchange非常简单，从名字也可以猜测出来，它会向所有的queue广播所有收到的消息。这正是我们的log系统需要的。

package com.rabbitmq.www.publish_subscribe;

import com.rabbitmq.client.BuiltinExchangeType;
import com.rabbitmq.client.ConnectionFactory;
import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;

public class EmitLog {

  private static final String EXCHANGE_NAME = "logs";
  private final static String HOST_ADDR = "172.18.112.102";

  public static void main(String[] argv) throws Exception {
    ConnectionFactory factory = new ConnectionFactory();
    factory.setHost(HOST_ADDR);
    Connection connection = factory.newConnection();
    Channel channel = connection.createChannel();
    //定义exchange
    channel.exchangeDeclare(EXCHANGE_NAME, BuiltinExchangeType.FANOUT);

    for(int i=0;i<=10;i++){
        String message = "helloworld"+i;
        
        //信息发送给申明的exchange
        channel.basicPublish(EXCHANGE_NAME, "", null, message.getBytes("UTF-8"));
        System.out.println(" [x] Sent '" + message );
    }

    channel.close();
    connection.close();
  }


}

package com.rabbitmq.www.publish_subscribe;

import com.rabbitmq.client.*;

import java.io.IOException;

public class ReceiveLogs {
  private static final String EXCHANGE_NAME = "logs";
  private final static String HOST_ADDR = "172.18.112.102";

  public static void main(String[] argv) throws Exception {
    ConnectionFactory factory = new ConnectionFactory();
    factory.setHost(HOST_ADDR);
    Connection connection = factory.newConnection();
    Channel channel = connection.createChannel();

    channel.exchangeDeclare(EXCHANGE_NAME, BuiltinExchangeType.FANOUT);
    //服务器为创建随机queue提供了一个无参数的queueDeclare()方来来创建一个非持久化的、独有的并且是自动删除的已命名的queue。
    String queueName = channel.queueDeclare().getQueue();
    //现在我们需要告诉exchange将消息发送到我们的queue中。 这种exchange和queue的关系称为绑定(binding)。
    channel.queueBind(queueName, EXCHANGE_NAME, "");

    System.out.println(" [*] Waiting for messages. To exit press CTRL+C");

    Consumer consumer = new DefaultConsumer(channel) {
      @Override
      public void handleDelivery(String consumerTag, Envelope envelope,
                                 AMQP.BasicProperties properties, byte[] body) throws IOException {
        String message = new String(body, "UTF-8");
        System.out.println(" [x] Received '" + message + "'");
      }
    };
    channel.basicConsume(queueName, true, consumer);
  }
}